Existing polar-loop transmitter circuits include a type of transmitter circuit in which: a signal transferred from a signal source is defined in the system of polar coordinates representing phases and amplitudes, and fed back to a power amplifier via a buffer amplifier and an amplitude comparator; and the power amplifier delivers a modulated signal (refer to, for example, “Polar-Loop Transmitter” by V. Petrovic and W. Gosling (Electronics Letters, Vol. 15, No. 10, pp. 286-2X, May 1979).
A type of polar-loop transmitter circuit having a power level thereof lowered is realized as a digital sigma transmitter to which an amplitude modulator is added (refer to, for example, “RF Micro Devices Polaris” (2 Total Radio Solutions)).